A Gassy Development Panel: Airflow Against Target
A bord-and-pillar development panel in a 2.8 m thick seam is advancing at 60 m a month with a continuous miner. Measured airflow in the general body at the last-cut through is 28 m3/s. Methane liberated from the exposed face and ribs at the present rate of advance averages 0.35 m3/s; during cutting, hand-held readings at the return of the working place touched 1.4%. The colliery manager wants to hold 60 m a month to meet the annual target and argues the average figure is 'within limits'. The ventilation officer wants the advance cut to 45 m a month but has no spare fan capacity - the main fan is at its duty point and the airways cannot be enlarged this quarter. No methane drainage boreholes exist on the panel.
1. Test the manager's claim using the dilution principle. What is the correct immediate decision?
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Required airflow to hold methane at 1.25% in the general body:
Q = 0.35 / 0.0125 = 28 m3/s
Available airflow is exactly 28 m3/s. So the margin is zero: the panel sits on the limit, and any fluctuation - a richer pocket, a slower air split, a fall obstructing the airway - pushes the general body over 1.25%.
The manager's 'average is within limits' argument fails for a second, independent reason: the peak reading during cutting was 1.4%, which is already above the 1.25% general-body limit at the return of the working place. An average is not the compliance test; the measured concentration at the point of measurement is.
Immediate decision: stop cutting, withdraw persons from the face to the intake side as required, and do not resume until airflow or gas emission is brought under control. Target is irrelevant against a face already over the limit.
2. The ventilation officer proposes 45 m a month. Estimate whether that buys an adequate margin, stating the assumption you are making and how you would verify it.
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Assume methane make is roughly proportional to advance rate over this range (more coal exposed per unit time, same seam gas content):
Emission ≈ 0.35 × (45/60) = 0.2625 m3/s
Required airflow = 0.2625 / 0.0125 = 21 m3/s, against 28 m3/s available - a margin of about 7 m3/s, roughly 25% of the general-body airflow, and the resulting concentration on the full 28 m3/s would be about 0.94%, comfortably below the limit.
The assumption - linear scaling with advance - is the weak point. Gas emission depends on seam gas content, friability, barometric swings and the number of exposed faces, none of which is constant. Verification: systematic sampling of the return over at least a full week at 45 m a month, including cutting and non-cutting shifts, plus a record of the highest reading each shift. If peaks still approach 1.0–1.1%, the advance must be cut further or drainage introduced; the calculation supports a trial, not a guarantee.
3. The manager's alternative is to run the existing fan harder. Assess that option, and state what must be achieved before 60 m a month is restored.
Read the worked answer
Running the fan harder is not available as an engineering answer here. The fan is already at its duty point; pushing beyond it moves the operating point up the characteristic curve, so added pressure yields diminishing and eventually negative airflow change, while raising the risk of motor overload and of recirculation through leakage. Airways that cannot be enlarged this quarter are the limiting resistance, and fan pressure cannot fix resistance cheaply. Presenting it as the low-cost option misstates the physics.
Restoring 60 m a month requires a change in the gas balance, not just more air. By this panel's own numbers, dilution at 60 m a month needs about 28 m3/s for the average case alone, with no allowance for peaks.
Before 60 m a month is restored, the panel should have:
- Methane drainage - in-seam or cross-measure boreholes to remove gas before it enters the airway, the only measure that changes the 0.35 m3/s itself;
- A verified airflow increase from airway enlargement or a re-split, demonstrated by a ventilation survey, not by a fan-speed setting;
- Continuous monitoring at the return with a triggered alarm and a written withdrawal procedure, so a 1.4% excursion produces action rather than a note in a register.